1. Field of the Invention
The present invention relates in general to video signal conversion systems and in particular to a video conversion system that selectively alters the dimensions of an image conveyed by an input video signal when converting the input video signal to an output video signal of differing format.
2. Description of Related Art
An image formed by K rows of pixels, with M pixels per row, may be represented by an M.times.K data matrix having elements of the form X(m,k) where each element represents the color of the mth pixel of the kth row of the image. Elements of such a data matrix can be sequentially encoded into analog signals and mixed with a subcarrier to produce a video signal for conveying the image data, for example, to a display monitor. There are many standards for video signals including, for example, devices which generate video signals following an RGB standard where each element X(m,k) of the data matrix has three values (R,G,B) representing intensities of red, green and blue components of the pixel's color. The PAL and NTSC television systems employ a video signal standard where each matrix element has three values (Y,U,V) where Y is luminance and the U,V data pair defines chrominance. The manner in which data is encoded into the analog video signal, the rate at which pixels are generated on a display (the scan rate), and the video signal subcarrier frequency may differ from one video signal standard to the next.
A video converter receives an input video signal and converts it to an output video signal. In the process, the converter may have to change several aspects of the input video signal including the color encoding scheme, the pixel dimensions of the images, the manner in which the pixel data is encoded into the video signal and the subcarrier frequency.
U.S. Pat. No. 5,526,055 issued Jun. 11, 1996 to Zhang et al describes two similar systems for converting an RGB-encoded video signal to a YUV encoded video signal: a system that is prior art to Zhang's system, and Zhang's system that is a modification to the prior art system. In the prior art system, the RGB video signal is digitized to produce a pixel data sequence which is re-encoded into YUV format and then stored in a memory. The resulting data sequence is then read out of the memory and supplied to an analog signal encoder. The encoder converts the data sequence to analog signals and mixes them with a clock signal oscillating at the output video signal subcarrier frequency to produce the output video signal. As the encoder mixes the analog video data with the subcarrier, it alters the pixel dimensions of the image to match the output video signal standards. Zhang modified the prior art video converter essentially by substituting a digital encoder for the analog encoder. Zhang's digital encoder is rather complex since it must include digital sine and cosine wave generators, multipliers and other components. In either video conversion system it is necessary to closely couple the subcarrier frequencies of the input and output video signals using relatively complicated timing systems employing phase lock loop circuits which may require discrete components.
What is needed is a video converter which can convert an input color video signal representing an input image to an output color video signal representing an output image where the pixel dimensions of the input and output images differ, where the color encoding schemes employed by the input and output signals differ, and where the subcarrier frequencies of the two signals differ. In addition, the video converter should not require the use of several phase lock controllers or complicated analog or digital encoders.